The increasing density of high performance integrated circuits (e.g., Gbit range DRAMs) is increasing the need for the use of materials with high dielectric constants in electrical devices and structures, such as capacitors. Generally, capacitance is directly related to the surface area of an electrode in contact with the capacitor dielectric, but is not significantly affected by the electrode volume. Current methods generally utilized to achieve higher capacitance per unit area increase the surface area/unit area by increasing the topography, such as in trench and stack capacitors using SiO2 or SiO2/Si3N4 as the dielectric. This approach becomes very difficult in terms of manufacturability for devices such as high capacity DRAM.
An alternative approach is to use a high permittivity dielectric material. Many perovskite, ferroelectric, or high dielectric constant (hereafter abbreviated HDC) materials, such as (Ba,Sr)TiO3 (BST), usually have much larger capacitance densities than standard SiO2—Si3N4—SiO2 capacitors. Various metals and metallic compounds, and typically noble metals such as Pt and conductive oxides such as RuO2, have been proposed as the electrodes for these HDC materials. To be useful in electronic devices, however, reliable electrical connections should generally be constructed which do not diminish the beneficial properties of these high dielectric constant materials.
As used herein, the term Ahigh dielectric constant@ means a dielectric constant of about 50 or greater at device operating temperature. HDC materials are useful for the fabrication of many electrical devices, such as capacitors. However, HDC materials are generally not chemically stable when deposited directly on a semiconductor substrate, so one or more additional layers are required to provide the electrical connection between the HDC material and the substrate. The additional layer or layers should generally be chemically stable when in contact with the substrate and also when in contact with the HDC material. Additionally, due to unit are constraints, high density devices (e.g., Gbit range DRAMs) generally require a structure in which the lower electrode is conductive from the HDC material down to the substrate. The deposition of an HDC material usually occurs at a high temperature (generally greater than about 500° C.) in an oxygen containing atmosphere. An initial electrode structure formed prior to this deposition should be stable both during and after this deposition, while subsequent electrode structures formed after this deposition need only be stable after this deposition.
There are a number of problems with the materials thus far chosen for the electrodes in standard thin-film (herein defined as generally less than 5 microns (μm)) applications. For example, although Pt is unreactive with respect to the HDC material, it has been found that it is difficult to use Pt alone as an initial electrode. Pt generally allows oxygen to diffuse through it and hence typically allows neighboring materials to oxidize. In addition, Pt also does not normally stick very well to traditional dielectrics such as SiO2 or Si3N4, and Pt can rapidly form a silicide at low temperatures.
Thus, a diffusion barrier is required to block the reaction between Pt and polysilicon. Such a diffusion barrier should have limited oxidation, as oxidation of the diffusion barrier will result in a rough Pt surface and high leakage current in the capacitor.
Materials such as TiN have been evaluated for this application. TiN is capable of blocking the diffusion of Pt and Si. However, TiN will oxidize to form TiO2, a process that is not self-limited and thus also results in a rough Pt surface and related problems.
Other materials such as TiAlN have also been proposed for this application. Oxidation of TiAlN will form a thin Al2O3 oxide on the surface. Due to the low diffusivity of oxygen and aluminum in Al2O3, the formation of the Al2O3 oxide is self-limited. Thus, the surface of TiAlN after oxidation will not have severe roughness. However, fabrication of TiAlN films present a number of other problems for deposition due to factors such as difficulty in maintaining composition uniformity throughout a sputtering target lifetime, difficulty in fabricating TiAlx target material without generating high particle counts, and the lack of a manufacturable CVD deposition solution.
Conductive oxides such as RuO2 generally also exhibit problems in standard thin-film structures. For examples, the electrical properties of the structures formed using these oxides are usually inferior to those formed using, e.g., Pt. Many thin-film applications require a small leakage current density in addition to a large capacitance per unit area. The leakage current is sensitive to many variables such as thickness, microstructure, electrodes, electrode geometry and composition. For example, the leakage current of lead zirconium titanate (PZT) using RuO2 electrodes is several orders of magnitude larger than the leakage current of PZT using Pt electrodes. In particular, it appears that the leakage current is controlled by Schottky barriers, and that the smaller leakage current with Pt electrodes appears to be due to the larger work function.